Generally, as shown in FIG. 1, an organic light emitting diode (OLED) being a flat panel display includes a number of pixels 100 which are arranged in the form of a matrix, each pixel 100 comprising three unit pixels, that is, a unit pixel 110R for embodying a red color (R), a unit pixel 120G for embodying a green color (G) and a unit pixel 130B for embodying a blue color (B).
The R unit pixel 110R includes a red electroluminescence (“EL”) device 115 including a red (R) light emitting layer, a driving transistor 113 for supplying a current to the red EL device 115, and a switching transistor 111 for switching the current supply from the driving transistor 113 to the red EL device 115.
The G unit pixel 120G includes a green EL device 125 including a green (G) light emitting layer, a driving transistor 123 for supplying a current to the green EL device 125, and a switching transistor 121 for switching the current supply from the driving transistor 123 to the green EL device 125.
The B unit pixel 130B includes a blue EL device 135 including a blue (B) light emitting layer, a driving transistor 133 for supplying a current to the blue EL device 135, and a switching transistor 131 for switching the current supply from the driving transistor 133 to the blue EL device 135.
Conventionally, the driving transistors 113, 123 and 133 of the R, G and B unit pixels 110R, 120G and 130B of an OLED have the same size, that is, the ratio W/L of the width W to the length L of the channel layer, and the order of the EL devices in the order of their luminous efficiencies is B, R and G unit pixel, where the B unit pixel has the lowest luminous efficiencies. Since the sizes of the driving transistors 113, 123 and 133 of the R, G, and B unit pixels 110R, 120G and 130B are same while luminous efficiencies of each R, G and B EL layer 115, 125 and 135 are different with one another, it was difficult to embody the white balance.
In order to embody the white balance, a relatively small quantity of current should be supplied to the EL device having high luminous efficiency, for example, green EL device, and a relatively large quantity of current should be supplied to the red and blue EL devices having low luminous efficiencies.
Here, since a current Id flowing to the EL device through the driving transistor begins to flow when the driving transistor is in the saturation state, the current is expressed as follows.Id=Cox μW{(Vg−Vth)}2/2L  (1)
Therefore, one of the methods for controlling the current flowing to the EL device in order to embody the white balance is to make the sizes of the driving transistors of the R, G and B unit pixels, that is, the ratio W/L of the width W to the length L of the channel layer, different and then to control a quantity of the current flowing to the EL devices of the R, G and B unit pixels. A method for controlling the quantity of current flowing to the EL device in accordance with the size of the transistor is disclosed in the Japanese Laid-open Publication No. 2001-109399. In the Japanese patent, the sizes of the driving transistors of the R, G and B unit pixels are differently formed in accordance with the luminous efficiency of the EL device in each R, G and B unit pixel. That is, the quantity of the current flowing to the EL device of the R, G and B unit pixels is controlled by making the size of the driving transistor of the green unit pixel having a high luminous efficiency smaller than those of the driving transistors of the red or blue unit pixels having relatively low luminous efficiencies.
Another method to embody the white balance is to make the dimensions of the light emitting layers of R, G and B unit pixels different, which is disclosed in the Japanese Laid-open Patent Publication No. 2001-290441. In this Japanese patent, the same luminance is generated from the R, G and B unit pixels by making the light emitting areas different in accordance with light emitting efficiencies of the EL devices of the R, G and B unit pixels. That is, the same luminance is generated from the R, G and B unit pixels by making the light emitting areas of the R unit pixel or the B unit pixel having lower luminous efficiencies relatively larger than the light emitting areas of the G unit pixel having a relatively high luminous efficiency.
However, in the conventional method for embodying the white balance described above, since the light emitting area of the unit pixel having low luminous efficiency among the R, G and B unit pixels is enlarged, or the size of the transistor of the unit pixel having low luminous efficiency among the R, G and B unit pixels is increased, the area occupied in each pixel is increased, and therefore it is not easy to apply the method to a high definition flat panel display (FPD).